In related arts, with regard to an automatic transmission in which one frictional engagement element is engaged in a plurality of different gearshift stages, there is a case where an engagement capacity (torque share) which the one frictional engagement element has differs according to each gear shift stage. Then, if the torque share is designed so as to meet a gear shift stage in which a large engagement capacity is required, there arises a problem that controllability of hydraulic pressure will deteriorate in a gear shift stage in which a small engagement capacity is required.
Thus, a Patent Document 1 discloses a technique in which a clutch of an automatic transmission has a plurality of fluid pressure chambers and a plurality of pistons whose pressure-receiving areas to receive a fluid pressure upon engagement are different from each other, and the fluid pressure chamber that supplies the fluid pressure is changed or switched according to a required engagement capacity.
In the following description, a ratio of the engagement capacity in each gear shift stage is described as a torque share ratio.